Downhole drilling operations commonly require a downhole tool to be actuated after the tool has been deployed in the borehole. For example, expandable reamers may be employed for enlarging subterranean boreholes. Conventionally, in drilling oil, gas, and geothermal wells, casing is installed and cemented to prevent the wellbore walls from caving into the subterranean borehole while providing requisite shoring for subsequent drilling operation to achieve greater depths. Casing is also conventionally installed to isolate different formations, to prevent cross-flow of formation fluids, and to enable control of formation fluids and pressure as the borehole is drilled. To increase the depth of a previously drilled borehole, new casing is laid within and extended below the previous casing. While adding additional casing allows a borehole to reach greater depths, it has the disadvantage of narrowing the borehole. Narrowing the borehole restricts the diameter of any subsequent sections of the well because the drill bit and any further casing must pass through the existing casing. As reductions in the borehole diameter are undesirable because they limit the production flow rate of oil and gas through the borehole, it is often desirable to enlarge a subterranean borehole to provide a larger borehole diameter for installing additional casing beyond previously installed casing as well as to enable better production flow rates of hydrocarbons through the borehole.
The blades in these expandable reamers are initially retracted to permit the tool to be run through the borehole on a drill string. At a depth (e.g., once the reamer has passed beyond the end of the casing), the expandable reamer may be actuated (e.g., hydraulically actuated). Actuation of the expandable reamer will enable the blades of the expandable reamer to be extended so the bore diameter may be increased below the casing.
One hydraulic actuation methodology involves wire line retrieval of a plug through the interior of the drill string to enable differential hydraulic pressure to actuate a reamer. Upon completion of the reaming operation, the reamer may be deactivated by redeploying the dart. However, wire line actuation and deactivation are both expensive and time-consuming in that they require concurrent use of wire line assemblies.
Another hydraulic actuation methodology makes use of shear pins configured to shear at a specific differential pressure (or in a predetermine range of pressures). For example, ball drop mechanisms involve the dropping of a ball down through the drill string to a ball seat. Engagement of the ball with the seat causes an increase in differential pressure which in turn actuates the downhole tool. The tool may be deactivated by increasing the pressure beyond a predetermined threshold such that the ball and ball seat are released (e.g., via the breaking of shear pins). However, such sheer pin and ball drop mechanisms are generally one-time or one-cycle mechanisms and do not typically allow for repeated actuation and deactivation of a downhole tool.
Other actuation mechanisms may utilize measurement while drilling (MWD) systems and/or other electronically controllable systems including, for example, computer controllable solenoid valves. Electronic actuation advantageously enables a wide range of actuation and deactivation instructions to be executed and may further enable two-way communication with the surface via conventional telemetry techniques. However, these actuation systems tend to be highly complex and expensive and can be severely limited by the reliability and accuracy of MWD, telemetry, and other electronically controllable systems deployed in the borehole.